14: Maintenance
14-14
PRC1099A-MS
3. Disconnect the 34-pin ribbon cable from the front of the Processor
board.
4. Remove the two screws securing the Power Amplifier and Antenna
Tuner modules tray and tilt the tray back to expose the Audio/Filter
board.
5. Disconnect the 10-pin J4 connector from the Audio/Filter board.
6. Disconnect the 4-pin cable from the Junction board.
7. Remove the four screws on the flange connecting the front panel to the
chassis.
8. Unsolder the coaxial cable connecting to the 50 ohm port on the front
panel.
9. Remove the front panel assembly from the chassis.
10. Reverse this procedure to install a new Front Panel assembly.
14.4
Semiconductor Servicing
There are two distinct classes of semiconductors used in the transceiver:
discrete devices, such as transistors and diodes, and monolithic integrated
circuits. You can obtain substantial information about the operation of the
transistors and diodes by measuring the voltage on the various leads. With
integrated circuits, however, there is no external access to much of the
circuitry, and it is often necessary to use the black box or substitution
approach to servicing.
14.4.1
Signal and Switching Diodes
You can check all diodes with an ohmmeter. They should show a low forward
resistance and a high reverse resistance. Check the circuit before making any
measurements, as the diode is frequently shunted by other components. This
requires you to lift one lead before measuring the resistance.
Many of the diodes are used as switches or gates. If the diode is operating
correctly, there will be a drop of approximately 0.7V across the diode junction
in the ON state.
14.4.2
Varactor Diodes
The varactor diode is designed to change capacitance across the
reverse-biased junction as the voltage applied to it changes. Varactors exhibit
the same resistance characteristics as signal diodes and can be checked in the
same way.
14.4.3
Bipolar Transistors
An out-of-circuit method of checking bipolar transistors is to consider the
base-emitter and the base-collector junctions as two separate diodes.